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Therapeutic hypothermia (TH) is the intentional reduction of a patient's core body temperature to reduce the likelihood of permanent damage to the brain and heart. Hypothermia decreases the body's demand for oxygen by slowing the cellular metabolic activity of the heart, brain, and other vital organs. Reduced metabolic activity can prevent destructive chemical reactions, such as free radical production, calcium shifts, and excitatory amino acid release, from occurring within cells.

Hypothermic treatments are potentially beneficial in the following situations:

* traumatic brain injuries

* neurogenic fever

* cerebral vascular accident

* postcardiac arrest care

* spinal cord injuries

* heart surgery

* neonatal encephalopathy

* preservation of donated organs

* hepatic encephalopathy

* subarachnoid hemorrhage.

The goal of TH is to reduce the core body temperature to 89.6° F to 93.2° F (32° C to 34° C) within 6 hours of the initial injury or event for 24 to 48 hours. Patients selected for TH are critically ill and should be quickly transferred to an ICU because the process to induce and maintain TH is labor intensive, requiring continuous monitoring to provide the patient with optimal neurologic and cardiovascular outcomes.

Ice packs, enclosed leak-proof packets filled with ice or cooled gel applied to strategic sites that emit body heat (such as the axilla, head, and groin), are labor intensive when used for TH, requiring the nurse to replace the cold packs hourly until the patient's core temperature reaches the desired goal. Place a barrier, such as a cloth or disposable pad, between the ice pack and the patient's skin to minimize the risk of localized tissue ischemia or frostbite that can occur from direct skin contact.

Cooling blankets are large specially designed pads that can be filled with cool water during the cooling phase and warm water during the rewarming phase. There are two hoses connected to the pad to continuously cycle temperature-controlled distilled water to and from the bedside electronic monitoring unit. Ensure the hoses stay securely latched to prevent the cooled water from leaking directly onto the patient's skin. Cooling blankets can visually obstruct access to the patient because of their size. Perform frequent skin assessments to assess for localized tissue ischemia and bleeding from invasive line sites.

Two liters of I.V. fluid cooled to 38.2° F (4° C) can be rapidly infused through a large bore peripheral I.V. to initiate TH. Infusing cooled I.V. fluid effectively dilutes normothermic blood, resulting in a rapid decrease in core body temperature. This method may be contraindicated in patients with diastolic dysfunction, a reduced cardiac ejection fraction, or those prone to fluid volume overload. It's also difficult to control the temperature of the chilled I.V. fluid because it begins to warm when removed from the refrigerated medication storage unit.

A CVCC, placed in the subclavian, femoral, or internal jugular vein, continuously circulates chilled saline solution through its cooling balloon lumen. As blood passes over the cooling lumen, it's directly cooled. The temperature of the coolant saline is constantly adjusted by a bedside monitor to maintain the desired core body temperature. After the patient has been rewarmed, the CVCC is typically removed after 96 hours due to associated venous thrombus formation risk.

PDC is the introduction of cold dialysate fluid into the peritoneal (abdominal) cavity to induce hypothermia. The dialysate is instilled into the abdominal cavity, flowing through the peritoneal dialysis catheter into the peritoneum by gravity. The catheter is then clamped for a specific amount of time—called the cooling dwell time—usually 20 minutes. The catheter is unclamped and the dialysate is drained to remove the cooled dialysate, wastes, chemicals, and extra fluid that pools in kidney failure patients. When administering PDC, ensure that the tubing accessing the catheter remains sterile to minimize the risk of peritonitis.

In gastric lavage, a nasogastric or orogastric tube is placed and 2 liters of ice water is instilled into the gastric tube with a 60 mL syringe to directly cool the stomach. Verify the gastric tube is in the correct location before instilling the ice water to prevent accidental instillation of the fluid into the lungs. This method is labor intensive and often utilized concurrently with external cooling methods such as a cooling blanket.

A neonatal cooling cap unit consists of a cooling monitor linking a supply and return line that circulates chilled fluid to the flexible cap that fits snugly against the baby's head. The cooling monitor can be adjusted at a selected temperature to either cool or warm the fluid infusing through the cap to cool or rewarm the brain. Ensure the supply and return lines are free from kinks, leaks, or damage.

A TECD consists of a transnasal cooling catheter similar to a nasal cannula, a pressurized coolant bottle, and a portable control cooling unit. The transnasal cooling catheter should be applied to the patient's nares similar to applying nasal cannula oxygen. The tubing line is then attached to the cooling unit that houses the pressurized gas filled coolant bottle. After the cooling unit has been programmed with the desired temperature, a steady stream of evaporated coolant gas is released through the catheter into the nasal cavity. This cools the extensive blood vessel network located within the nasopharyngeal area and the central facial bones, resulting in a rapid reduction in brain temperature.

ECMO is lifesaving support for the most severe forms of acute heart and/or lung failure. The six components of ECMO include a venous cannula, an arterial cannula, an oxygenator, a heat exchanger, an ECMO circuit, and a reservoir bladder (see A closer look at ECMO). ECMO can be used on infants, children, and adults with severely damaged hearts or lungs, acute lung injury patients who are failing ventilator support or during cardiac bypass surgeries, or patients experiencing profound systemic accidental hypothermia. Although ECMO can be used to induce TH, it's rarely utilized because of the extensive set-up time, labor intensive maintenance, and potential risk of complications.

Physiologic effects

A multidisciplinary healthcare team approach is needed to effectively manage a patient during the induction, maintenance, and rewarming phase of TH. Measure the patient's core body temperature continuously by placing a temperature sensor in the rectum or esophagus, or by placing a urinary catheter equipped with temperature sensors on the balloon tip that measures bladder temperature.

The following are some of the physiologic affects that can be seen with TH.

Brain. TH lowers the incidence of cerebral anoxia by reducing the cerebral metabolic rate. This induces the brain into a hibernation-like state that requires minimal oxygen, energy, and glucose needs. For every 1° C in body temperature, the cerebral metabolic rate is decreased by 6% to 10%. TH decreases elevated intracranial pressure by relieving pressure on compressed cerebral blood vessels and brain tissue, which can prevent anoxia, ischemia, and permanent neurologic deficits. When performing a neurologic assessment during TH therapy, expect to see a decreased level of consciousness, sluggish pupils, decreased cough and gag reflex, slurred speech, and decreased physical response to painful stimuli.

Heart. When TH is initiated, the patient may experience cardiac irritability and ECG changes. You may initially see sinus tachycardia (heart rate greater than 120); after TH is achieved, the patient may experience bradycardia (heart rate less than 60), atrial fibrillation, or atrial flutter. Perform a 12-lead ECG every 6 hours and maintain the patient on continuous ECG monitoring to assess for changes in heart rhythm, rate, and waveform (such as prolonged PRI, prolonged QTI, or T wave inversion).

Lungs. As the patient's core body temperature is decreased, the lungs become less compliant as the bronchioles dilate and the tissue becomes stiff. TH also causes a left shift of the oxy-hemoglobin dissociation curve, which means that hemoglobin has a higher affinity for oxygen. During the induction and maintenance phase, you typically won't see the patient initiate any independent breaths because of the usage of paralytic medications. You may notice a decreased independent respiratory rate during the rewarming phase because paralytic medications are commonly weaned off or the set ventilator rate may be programmed to administer a high number of breaths per minute.

Kidneys. TH inactivates antidiuretic hormone (ADH), which can result in profound cold-induced diuresis and loss of electrolytes as fluid shifts intravascularly from the peripheral circulation to the central core. You may notice a large increase in urine output; monitor fluid losses closely. This usually corrects itself after the patient is rewarmed. The patient may require additional boluses of I.V. fluid to prevent dehydration and cardiovascular collapse. During all phases of TH therapy, monitor serum labs for electrolyte disturbances.

Blood. TH causes numerous hematologic changes. The pH of the blood increases 0.016 for every 1° C the core body temperature is lowered. Consult with other members of the healthcare team to ensure a baseline arterial blood gas level is performed before the initiation of TH therapy. The patient's hematocrit can increase up to 2% for every 1° C increase in core temperature. This places the patient at risk for thrombus formation as the viscosity of the blood increases. Ensure the patient has DVT prophylaxis, such as sequential compression devices, antithrombolytic stockings, or anticoagulant therapy. Numerous serum electrolytes are often affected, such as sodium, potassium, magnesium, and phosphate. During the induction and maintenance of TH, potassium shifts into cells, resulting in serum hypokalemia. Monitor serum electrolyte test results closely.

Pancreas. During TH, the pancreas secretes less insulin to the body's vital organs and tissues. This can result in hyperglycemia during TH therapy; monitor blood glucose levels frequently. The patient may require medical interventions such as an insulin drip. TH can induce insulin resistance at the cellular level, making it difficult to maintain tight glycemic control.

Warm up time

TH patients should be rewarmed slowly at a rate of 0.50° C over a 12-hour period to minimize complications. When rewarming has been initiated, the patient is at risk for cardiac arrhythmias and profound hypotension as the heat begins to cause venous dilation. Continue to monitor both the ECG and vital signs closely.

As the patient is rewarmed, he or she may become febrile as the hypothalamus adjusts to the discontinuation of treatment. The patient may also experience an increase in ADH release, and urine output may decrease. If blood viscosity decreases, the patient can be at risk for bleeding complications. Monitor the serum prothrombin time (PT), partial thromboplastin time (PTT), and international normalized ratio (INR) and all invasive lines for signs of active bleeding.

Patients are usually maintained in the ICU for a minimum of 72 hours after they've been successfully rewarmed without complications or before making end-of-life decisions when a poor prognosis is suspected.

A much-needed rest

TH slows the body's metabolic clock, minimizing cell damage and allowing needed time to rest and heal. Multidisciplinary teams are utilizing TH to save lives, as well as preserve patients' quality of life.

A closer look at ECMO

A large bore cannula is placed in a large vein, such as the jugular or femoral vein, draining the blood into the ECMO reservoir bladder. It's then pumped through the ECMO circuit by a roller pump. The pump provides enough blood flow to support the workload of a failing heart. The roller pump gently forces the blood into the oxygenator that infuses the blood with oxygen and removes carbon dioxide. Blood is then pumped into the heat exchanger, where it can be warmed or cooled to lower the core body temperature. It's returned to the body via the same venous cannula or through an optional arterial cannula in the common carotid artery. The cooled blood lowers the body temperature to the desired goal and slows the heart's metabolic activity, which reduces the likelihood of permanent damage to the heart tissue.

This procedure allows the heart and lungs to rest and heal. ECMO can also be utilized to raise the core body temperature in patients experiencing profound systemic accidental hypothermia. Blood is slowly warmed as it filters over the heat exchanger. After the warmed blood is returned to the body, the patient's body temperature slowly begins to rise.

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